NEW YORK (CBSNewYork/AP) — An inmate whose wrongful-conviction claim has been championed by actor and activist Martin Sheen is asking a court to throw out his New York City murder conviction after prosecutors declined to do so.

Jon-Adrian Velazquez is serving a life sentence for shooting retired police officer Albert Ward in Harlem in 1998.

Attorney Robert Gottlieb filed a motion Thursday for a new trial based on new evidence, saying there’s not enough proof left in the case after some eyewitnesses backtracked.

Man Asks Court To Nix Conviction In 1998 Murder Case

“The motion is based on the unreliability of the original eyewitness identification as well as the new evidence — witnesses who have stepped forward very courageously and have indicated that another man has admitted to them and to others that he is the killer,” Gottlieb told 1010 WINS.

“We have proven that he is innocent!” said Maria Velazquez, the convicted man’s mother. “It’s been a cruel joke that the DA has played on this family because we trusted in him to bring us justice!”

Prosecutors re-investigated the case of Jon-Adrian Velazquez, but decided there was insufficient proof to overturn his conviction.

The case began in Harlem in 1998 when Ward, an ex-cop who ran a gambling den, was gunned down during a robbery, CBS 2’s Aiello reported.

Jon-Adrian Velazquez, then 21 and the father of two, was convicted after several witnesses who picked him out of a lineup said he was the gunman.

“Those eyewitnesses are absolutely worthless,” Gottlieb said.

Gottlieb said he presented evidence showing the witnesses are unreliable, and gave the DA a solid lead that a man who looks like the original police sketch — a man named “Moustapha” — is the real killer.

“The Conviction Integrity Unit turned out to be a joke,” Gottlieb said.

The district attorney’s office is strongly defending its review of the case, and pointing out a big problem with the theory a man named Moustapha is the real killer.

A spokeswoman said, “Our investigation found [Moustapha] was not even in New York state on the day of the murder,” and that “it is beyond baffling… these lawyers…would vilify the district attorney’s office now that the result is not what they had hoped for.”

On Thursday, Jon-Adrian Velazquez’s attorneys filed a formal appeal, putting it all in the hands of a judge.

The DA’s Conviction Integrity Unit has done a full review of 12 cases, leading to the overturning of two convictions.